1. Field of the Invention
The present invention relates to an image reproducing apparatus and, more particularly, to an image reproducing apparatus for concealing undecodable data of a video signal which was high efficiently encoded and for reproducing such data.
2. Related Background Art
An orthogonal transformation encoding method is known as a technique for highly efficiently compressing and encoding an image signal. According to such a method, after the image signal has been divided into blocks every predetermined number of pixels in a lump, an orthogonal transformation such as a discrete cosine transformation (DCT) or the like is executed. A quantization, an entropy encoding or the like is executed on coefficients after completion of the transformation.
According to such a method, since the encoding is executed on a block unit basis, when an error occurs in the encoding data, a code error occurs on a block unit basis. Since a variable length code is frequently used, a decoding error is propagated over a plurality of blocks, so that it results in a large deterioration of the image.
An interframe interpolation in which an error block is replaced by the image of a previous frame is known as a means for obtaining a reproduction image by interpolating and concealing such an error block.
FIG. 1 shows a concealing operation by interframe interpolation. Now, assuming that a block X of a frame #n cannot be reproduced, the interframe interpolation is realized by replacing the data of the block X by the reproduction data of a block A' at the same position on a screen of a previous frame #n-1 and by reproducing the data.
According to such an interframe interpolation, however, when a movement of an image is hard, the interpolation data is deviated from the image data of a peripheral block in the same field with respect to the time and differs from the image data. Therefore, when the data of the error block is replaced by the interpolation data, it becomes a visually large obstruction.
A method of using image data in the same field as interpolation data is also considered. In this case, however, although there is a correlation as compared with that of the interframe interpolation with regard to the time, a correlation is contrarily low with respect to the space.
Therefore, there is also considered a method whereby a movement of an unreproducible block is estimated by the image data of an upper block of the unreproducible block. The interframe interpolation and the intrafield interpolation are switched in accordance with the movement. In this case, however, since the movement of the unreproducible block is estimated by only the image data of the upper block, it is likely to erroneously estimate the movement and there is a possibility that the error data is interpolated by data of a low correlation.